Method and apparatus for amplifier output biasing for improved overall temperature stability

ABSTRACT

An amplifier output is biased to optimize performance characteristics such as gain and output voltage. A temperature-independent current is subtracted from a temperature-dependent current. The difference is injected at the amplifier output to bias the amplifier output such that performance characteristics are enhanced. An additional amplifier stage may be used to prevent the bandwidth performance of the amplifier from being affected by the current injection.

BACKGROUND

This invention relates generally to a method and apparatus for amplifierbiasing. More specifically, this invention relates to a method andapparatus for optimizing the temperature dependence of variousperformance parameters of an amplifier.

The design of a typical bipolar amplifier requires trade-offs between anumber of characteristics, such as noise voltage, gain, and outputquiescent voltage over a range of various environmental and processconditions. Typically, designs which require low noise must use anundegenerated common-emitter amplifier (or differential pair fordifferential designs) with resistive loading. An example of such anamplifier is shown in FIG. 1 which depicts an amplifier stage 100including a transistor QI fed at the base by a voltage VIN and connectedat the collector to a supply voltage VCC, via a resistor R. Connected tothe emitter of the transistor is a current source IEE. The outputvoltage is given as VOUT=VCC−ICC*R, and the gain is roughly given byAV=IEE*R/VT, where VT=kT/q, and k is Boltzmann's constant, T istemperature, and q is the electron charge. Ignoring the temperaturedependence of R, dAV/dT is proportional to dIEE/dT−dT/dT, and dVOUT/dTis proportional to dICC/dT. Therefore, in order to minimize thevariation of the gain AV and the output voltage VOUT over temperature,IEE must be proportional to temperature, while ICC must be constant.

Typically, this is not possible due to the fact that ICC=IEE (ignoringbase current errors). Thus, a designer is typically forced to choose tomaintain either constant gain AV or constant output voltage VOUT overtemperature by appropriately defining the temperature dependence of IEE.A proportional-to-absolute-temperature (PTAT) current reference can beused to define IEE and ICC to maintain a constant gain AV overtemperature, or a bandgap (BGAP) current reference can be used to defineICC and IEE to maintain a constant output voltage VOUT over temperature.

There have been various attempts to optimize performance characteristicsof amplifiers. Some of these attempts have been directed at optimizingthe gain by injecting a current at the amplifier output. For example,U.S. Pat. No. 5,798,660 describes how the gain of a CMOS differentialamplifier stage may be enhanced by the injection of additional currentat the drains of the differential pair. U.S. Pat. No. 5,436,594describes a current source that biases a differential amplifier tocontrol the gain of the amplifier.

None of these past attempts have independently optimized the temperaturedependency of multiple parameters, such as gain and output voltage.

There is thus a need for a technique for amplifier output biasing thatsimultaneously minimizes temperature variations of various performanceparameters.

SUMMARY

It is therefore an object of the present invention to simultaneouslyoptimize the temperature dependence of various performance parameters.

According to an exemplary embodiment, this and other objects are met bya method and apparatus for amplifier output biasing. Atemperature-dependent current is generated, and atemperature-independent current is generated. Thetemperature-independent current is subtracted from thetemperature-dependent current. The difference is injected at theamplifier output to optimize the temperature dependence of performancecharacteristics such as gain and output voltage. The amplifier may be adifferential amplifier. An additional amplifier stage may be used toprevent the current injection from affecting the bandwidth performanceof the amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the invention will becomeapparent by reading this description in conjunction with theaccompanying drawings, in which like reference numerals refer to likeelements and in which:

FIG. 1 illustrates a conventional apparatus for amplifier outputbiasing;

FIG. 2A illustrates an apparatus for amplifier output biasing accordingto an exemplary embodiment;

FIG. 2B illustrates an apparatus for amplifier biasing including anadditional stage according to an exemplary embodiment;

FIG. 2C illustrates an apparatus for biasing a differential amplifieroutput according to an exemplary embodiment; and

FIG. 3 illustrates a method for amplifier output biasing according to anexemplary embodiment.

DETAILED DESCRIPTION

According to exemplary embodiments, a method and apparatus are providedfor amplifier output biasing. Different performance characteristics canbe optimized by generating a temperature-dependent current and atemperature-independent current and injecting the difference betweenthese currents at the amplifier output.

FIG. 2A illustrates an apparatus for biasing an amplifier outputaccording to an exemplary embodiment. The device 200 ensures that boththe gain AV and the output voltage VOUT remain stable over temperaturevariations. The bias currents IEE and ICC are independently optimizedthrough the use of current subtraction. A first current source I1 isused to define IEE, while a second current source I2 defines ICC. Afirst current generator I1 generates a current that istemperature-dependent, e.g., a current that is proportional to absolutetemperature (IPTAT), and a second current generator I2 generates acurrent that is fairly constant and temperature-independent, e.g., aband gap current (IBGAP). These currents may be generated, e.g., by biascurrent generators. Although shown separately, the current sources I1and I2 may be included in the device 200.

As shown in FIG. 2A, a first current mirror mirrors the current for IEEthat is temperature-dependent, to cancel out the VT term in the gainequation. A second current mirror mirrors the current IEE again. A thirdcurrent mirror mirrors the current I2 and generates a current for ICCthat has a negligible temperature dependence. A fourth current mirrorhas its input connected to the output of the second current source I2and has its output connected to the output of the second current mirrorand the third current mirror. The output of the fourth current mirror isthus proportional to the quantity IEE−ICC. This current is mirrored by afifth current mirror, the output of which is connected to the amplifieroutput node, e.g., the collector. Since the amplifier emitter current isdefined as IEE, this forces the resistive load current to be equal toIEE−(IEE−ICC)=ICC. Thus, the injection of the current into the amplifierproduces an output bias current that is not current dependent, so thatthe output voltage VOUT is independent of temperature.

While it has been assumed that R is constant over temperature, if Rvaries with temperature, this can be taken into account in the IEE andICC current references, such that the current and resistor temperaturedependencies cancel out. The process variation of the gain and voltageoutput can be minimized by using the same type of resistor for R as usedin the IEE and ICC reference circuits.

According to an exemplary embodiment, an additional amplifier stage maybe used to prevent the bandwidth of the amplifier from being affected bythe load of IEE−ICC. For example, a common base stage may be added toform a cascode configuration, as shown in FIG. 2B. The common base stageshown includes a transistor Q2 with a base connected to a voltagereference VREF, an emitter connected to the collector of Q1, and acollector connected to the resistor R at the amplifier output. Thecommon base stage isolates the injected current from the output, so thatadding the current injection load does not affect the bandwidthperformance of the amplifier.

The apparatus for biasing an amplifier output may be used for biasing adifferential amplifier, as shown in FIG. 2C. In FIG. 2C, a device 255insures that both the gain and the output voltage remain stable overtemperature variations. By way of example, the device shown in FIG. 2C,includes a temperature-dependent current source, e.g., aproportional-to-absolute-temperature current source Iptat, that ismirrored by a first current mirror to generate a current 2*Iee in orderto cancel out the effect of the VT term in the gain equation. A secondcurrent mirror generates a current Iee. A temperature-independentcurrent source, e.g., a bandgap reference current source Ibgap, ismirrored by a third current mirror, and again, by a fourth currentmirror to produce a current Icc which has negligible temperaturedependence. A third current mirror generates a current Iee. A fifthcurrent mirror has its input connected to the second current source andits output connected to the fourth current mirror and the second currentmirror and thus generates a current proportional to Iee−Icc. Sixth andseventh current mirrors mirror this current and deliver it to thecollectors of Q3 and Q4.

Although Iptat and Ibgap are used as current references in the examplesdescribed above, the current references are not so restricted. Anytemperature-dependent current source may be used in place of Iptat, andany substantially temperature-independent current source may be used inplace of Ibgap.

Additional stages may be connected to the collectors of Q3 and Q4,isolating the collectors from the output. For example, as shown in FIG.2C, transistors Q5 and Q6 may be added in a cascode configuration withtheir bases connected to a voltage reference Vref, their emittersconnected to the collectors of transistors Q3 and Q4, respectively, andtheir collectors connected to the resistors R at the amplifier output.It will be appreciated that the common base stage, althoughadvantageous, is not necessary.

The configurations of current mirrors shown in FIGS. 2A-2C are onlyexamples of how current injection may be implemented. Instead of thecurrent mirrors shown current sources can be used.

FIG. 3 illustrates an exemplary method for amplifier biasing. The methodbegins at step 300 at which a temperature independent current issubtracted from a temperature dependent current. These currents may begenerated separately or as part of the process of amplifier biasing. Atstep 310, the difference is injected at the amplifier output, thusproviding the desired output bias current.

According to exemplary embodiments, a method and apparatus are providedfor biasing an amplifier output to enhance performance characteristics.The method and apparatus described above may be used for, e.g., widebandwidth amplifiers, e.g., a read amplifier operating in the 100 MHz-1GHz range.

Although described in terms of amplifiers formed of one or moretransistors, the invention is not so limited. It will be appreciated bythose of ordinary skill in the art that this invention can be embodiedin other specific forms without departing from its essential character.The embodiments described above should therefore be considered in allrespects to be illustrative and not restrictive.

I claim:
 1. An apparatus for biasing an amplifier output, comprising: asubtraction circuit for subtracting a temperature-independent currentfrom a temperature-dependent current to produce a difference current;and a current injection circuit for injecting the difference current atthe output of the amplifier to bias the amplifier output and enhanceperformance characteristics.
 2. The apparatus of claim 1, wherein theperformance characteristics include gain and output voltage.
 3. Theapparatus of claim 1, wherein the amplifier is a differential amplifier.4. The apparatus of claim 1, further comprising an additional amplifierstage for preventing the bandwidth performance of the amplifier frombeing affected by the current injection.
 5. The apparatus of claim 1,wherein the temperature-independent current is generated by a band gapcurrent reference.
 6. The apparatus of claim 1, wherein thetemperature-dependent current is generated by aproportional-to-absolute-temperature current reference.
 7. The apparatusof claim 1, wherein the amplifier comprises a transistor, and thecurrent is injected into a collector of the transistor.
 8. The apparatusof claim 3, wherein the amplifier comprises two transistors, and thecurrent is injected into the collectors of the transistors.
 9. A methodfor biasing an amplifier output, comprising the steps of: subtracting atemperature-independent current from a temperature-dependent current toproduce a difference current; and injecting the difference current atthe output of the amplifier to bias the amplifier output and enhanceperformance characteristics.
 10. The method of claim 9, wherein theperformance characteristics include gain and output voltage.
 11. Themethod of claim 9, wherein the amplifier is a differential amplifier.12. The method of claim 9, further comprising a step of processing theoutput of the amplifier through an additional amplifier stage forpreventing the output bandwidth performance from being affected by thecurrent injection.
 13. The method of claim 9, wherein thetemperature-independent current is generated by a bandgap currentreference.
 14. The method of claim 9, wherein the temperature-dependentcurrent is generated by a proportional-to-absolute-temperature currentreference.
 15. The method of claim 9, wherein the amplifier comprises atransistor, and the step of injecting comprises injecting a current intoa collector of the transistor.
 16. The method of claim 11, wherein theamplifier comprises two transistors, and the step of injecting comprisesinjecting currents into the collectors of the transistors.